[hail][bugfix] Fix divide by zero error in hl.concordance (#7976)

Fixes #7974

hail/python/hail/methods/qc.py

@@ -459,7 +459,8 @@ def n_discordant(counter):
         total_conc = [x[1:] for x in glob[1:]]
         on_diag = sum(total_conc[i][i] for i in range(len(total_conc)))
         total_obs = sum(sum(x) for x in total_conc)
-        info(f"concordance: total concordance {on_diag/total_obs * 100:.2f}%")
+        pct = on_diag/total_obs * 100 if total_obs > 0 else float('nan')
+        info(f"concordance: total concordance {pct:.2f}%")
 
     per_variant = joined.annotate_rows(concordance=aggr)
     per_variant = per_variant.select_rows(concordance=concordance_array(per_variant.concordance),

hail/python/test/hail/methods/test_qc.py

@@ -212,6 +212,13 @@ def make_mt(rows):
                       n_discordant=0),
         ]
 
+    def test_concordance_no_values_doesnt_error(self):
+        dataset = get_dataset().filter_rows(False)
+        _, cols_conc, rows_conc = hl.concordance(dataset, dataset)
+        cols_conc._force_count()
+        rows_conc._force_count()
+
+
     def test_filter_alleles(self):
         # poor man's Gen
         paths = [resource('sample.vcf'),